An example of the above-noted hydraulic system for the work vehicle is disclosed in Japanese Unexamined Patent Application Publication No. 2000-085597 (JP 2000-085597 A) (Patent Document 1) With this hydraulic system, in a steering operation, the entire amount of oil from a fixed displacement hydraulic pump actuated by drive power from the engine is supplied preferentially to a hydraulically operated power steering device. In a non-power steering operation, on the other hand, oil from the hydraulic pump is supplied preferentially through a preferential flow-dividing valve to a hydraulically operated elevation actuating device for vertically moving a work implement (e.g. elevation control valve and lift cylinder) and a hydraulically operated rolling actuating device for rolling and actuating the work implement (e.g. rolling control valve and rolling cylinder).
Since the fixed displacement pump is used for the hydraulic pump in such a conventional hydraulic system, a discharge rate of the hydraulic pump when actuating at low rotational speed is set to a great value in order to secure an oil flow rate required for actuating the power steering device and an oil flow rate required for actuating the elevation actuating device and the rolling actuating device even when the rotational speed of the engine is low. Therefore, when the rotational speed of the engine is increased, an amount of oil more than necessary is supplied to the power steering device, etc. This results in waste of energy and increased oil temperature. Thus, there is room for improvement from the viewpoint of saving energy.
In addition, since the elevation actuating device cannot be operated during the power steering operation, the steering operation and the elevation operation of the work implement cannot be concurrently performed, such as raising the work implement when starting a turn for executing the steering operation or lowering the work implement when finishing the turn, in order to make a turn in a verge of a ridge during a cultivating operation with a rotary tiller being connected to a rear portion of the work vehicle. Thus, there is room for improvement in operability.
Further, when the elevation actuating device and the rolling actuating device are concurrently actuated, oil is supplied preferentially to the rolling actuating device under the action of the preferential flow-dividing valve, which might cause a retarded operation of the elevation actuating device. In order to avoid such an inconvenience, it is necessary to increase the discharge rate of the hydraulic pump, which makes it further difficult to save energy.